To print a printing substrate (made of paper, for example) it is known to generate image-dependent charge images on a charge image carrier (for example a photoconductor drum) that correspond to the images to be printed, and comprised of regions that are to be inked and regions that are not to be inked. The regions of the charge images that are to be inked can be developed with a liquid developer containing at least toner and carrier fluid. The developed images—the toner images—are subsequently transfer-printed onto the printing substrate and fixed there.
A method for such an electrophoretic liquid development in digital printing systems is known from WO2007/57387 (US 2008/279597 A1), for example. A carrier fluid containing silicon oil, hydrocarbons or vegetable oils, which carrier fluid has ink particles (toner) dispersed in it, is thereby used as a liquid developer. Among other things, a charging station, an element for graphical exposure (character generator), a developer station to develop the charge images with toner, a transfer station to transfer-print the developed charge images (the toner images) onto a printing substrate and an element to clean the photoconductor drum after the transfer-printing are arranged along a charge image carrier (for example the photoconductor drum) in the printing apparatus according to WO2007/57387.
The developer station according to WO2007/57387 has a reservoir container for the liquid developer, a chamber blade or scraper with a rotating anilox or screen roller; and a rotating developer roller. The anilox roller takes up the liquid developer of the chamber blade and transfers this to the developer roller, which directs the liquid developer past the photoconductor drum to develop the charge images into toner images. The toner images are supplied from the charge image carrier via the transfer station to the printing substrate and transfer-printed onto this. For this a rotating transfer roller can be used that accepts the toner images (embedded in carrier fluid) from the charge image carrier and supplies them to the printing substrate. In the transfer printing station the toner images are transfer-printed onto the printing substrate using an electrical field and are subsequently fixed there.
In the generation of toner images using carrier fluid and liquid developer with toner, a film made up of liquid developer is supplied to the charge image carrier. After the inking of the charge images with the liquid developer, the toner images (embedded in a film made of carrier fluid) are transfer-printed onto the printing substrate in the transfer station. In the following explanation this film made of liquid developer or carrier fluid with toner images is uniformly called liquid film.
Roller pairs with rotating rollers as transport units can be used to transfer the liquid film. The layer thickness of the transported liquid film thereby depends on, among other things, the surface velocities of the rollers of the respective roller pair and the contact pressure force of the rollers on one another. Problems arise when the surface velocities of the rollers of the roller pairs change, as this is the case when a printing device approaches its final speed. If in this case the printing of the printing substrate should already take place before reaching the final speed—thus in the start phase of the printing operation—the print images are of reduced print quality.